Optical fiber systems operating at the 1500 nm wavelength region have received much attention in recent years. Planar optical waveguide devices and circuits are essential for these fiber communication systems, especially in metropolitan area networks and local area networks. A wide range of optical waveguide devices, such as splitters, couplers, switches and modulators are required and many designs are known for such devices.
However, the overall system performance can be degraded as a consequence of losses. Losses may occur in particular either within individual devices or at the junctions and interconnections between devices. Such losses may result from waveguide imperfections, coupling losses, propagation losses, and losses due to mismatch between waveguide and fiber modes. In order to compensate for such losses optical systems may be provided with optical amplifiers and such optical amplifiers are of major importance and play a significant role in optical communications systems.
A known technique for providing optical amplification in an optical waveguide is to dope the waveguide core with rare earth ions. Erbium (Er3+) ions are known to be particularly useful in this context because the 4I13/2−4I15/2 transition near 1540 nm wavelength matches one of the fiber low-loss windows. For this reason erbium-doped fiber amplifiers have already been implemented in optical fiber systems, and erbium-doped waveguide amplifiers have been fabricated using glass and crystal host materials.
Polymeric materials have many advantages for use in optical waveguide devices. These advantages include low costs, high packaging density and simple processing steps. Existing techniques for fabricating such devices include reactive ion beam etching, photobleaching, ion-implantation processes, and conventional lithography followed by etching. However, these known methods have the disadvantage that they involve many processing steps and can lead to a long fabrication time and low yield.
A further problem with existing optical amplifiers based on polymeric materials is that compared with inorganic host materials such as glass and crystal it is comparatively difficult to dissolve rare earth ions in polymeric materials, because most of the rare earth ions are in inorganic salt forms that do not mix well with polymers and coagulation occurs. Polymer-based optical amplifiers have been doped with Nd3+ as for example in R. T Chen, M. Lee, S. Natarajan, C. Lin, Z. Z. Ho and D. Robinson, “Single-Mode Nd3+-doped graded-index polymer waveguide amplifier”, IEEE Photon. Technol. Lett. Vol. 5, p. 1328-1331 (1993) which described NdCl3-doped photolime-gelatin, and G. Karve, B. Bihari and R. T. Chen, “Demonstration of optical gain at 1.06 μm in a neodymium-doped polyimide waveguide”, Appl. Phys. Lett. Vol. 77, p. 1253-1255 (2000) describing neodymium-doped polyimide. The doping of polymers with other rare-earth ions such as erbium has, however, been much harder to achieve.